The invention relates to a circuit arrangement for a position-sensitive radiation detector, which, when electromagnetic radiation is received, supplies two output currents I.sub.1, I.sub.2, the amplitude of which depends upon the coordinates of the point of incidence of the electromagnetic radiation. The circuit arrangement, to which the output currents I.sub.1, I.sub.2 are supplied, generates a signal that depends only on the particular coordinate and not on the intensity of the incident radiation. The circuit arrangement includes (1) a first device, to which an output current can be supplied and which generates a signal that is proportional to the product of the output current and time, and (2) a second device, to which an output current is supplied and which generates a signal that is proportional to the product of the output current and time.
In numerous fields of technology, it is necessary to know exactly the point of incidence of electromagnetic radiation (hereinafter "EMR"). For example, the point of incidence of an electron beam on the target of an electronic beam vaporizer source must be determined accurately, in order to be able to control it so that as uniform a surface temperature as possible is achieved over a particular area and, with that, a constant evaporation rate over this area. A strictly visual control is unsuitable for a fully automatic control system. Components are therefore used which respond to a particular radiation. These components react, for example, to the UV light or to the X-rays, which are formed when an electron beam strikes a target. In contrast to conventional photocells, light-dependent resistances or the like, which change their electric state as a function of the total amount of the incident radiation, and which are therefore not in a position to indicate space coordinates, the aforementioned components must have properties which make it possible to allocate space coordinates.
So-called lateral diodes, for example, are such components. These are position-sensitive semiconductor photodiodes of large surface, which utilize the lateral photoeffect. These photodiodes are also referred to as "PSD's" (position sensitive detectors). Provided that a suitable bias has been selected, unidirectional lateral diodes, a portion of the sensitive surface of which is struck by electromagnetic radiation, supply two output currents, the sum of which is proportional to the incident radiation and the difference between which is proportional to the intensity and proportional to the positional coordinates of the center of gravity of the incident radiation. The difference between the currents or a voltage proportional to this difference is referred to as a positional signal that has not been normalized. For many applications, however, the normalization of this signal, that is, the formation of a signal that is exclusively proportional to position, is required.
Various circuit arrangements have already been proposed to achieve such a normalization (see FIGS. 45 and 6 of the European Patent No. 85114404.8--Publication No. 0 184 680). These circuit arrangements are based on the fundamental concept that, by dividing the sum of the currents by the difference between the currents or the reverse, the influence of the intensity of the irradiation is eliminated and only the influence of the space coordinates is left in the current difference. This division is carried out by means of an analog divider, which divides a voltage that is proportional to the difference between the currents by a voltage that is proportional to the sum of the currents.
With such an arrangement, it is a disadvantage that the maximum permissible fluctuation range of the sum of the currents or of the intensity of the radiation striking the lateral diode is limited by the dynamics of the analog divider. Systems with high dynamics and operating according to the principle described above can be realized only by using expensive, precision components; otherwise, it is necessary to switch over the amplification factor of the preselectors. It is a further disadvantage of the known circuit arrangement that the achievable position resolution falls as the sum of the currents or the radiation intensity decreases.
Moreover, evaluation electronics for a differential photodiode sensor with the reciprocal transfer process of a capacitor and a pulse-width modulated output voltage are known, for which only one capacitor is used for two photodetectors for the reciprocal transfer process by a luminosity-proportional charging current of the photodiodes (German Patent Publication No. 3,531,378). For this system of evaluation electronics, the positional signal is obtained from the variable pulse-width repetition ratio of a pulse-width modulated square-wave voltage; that is, the positional signal is represented by the d.c. voltage fraction of this square-wave voltage, obtained by deep-pass filtration.
It is a disadvantage of this known system of evaluation electronics that the threshold frequency of the evaluation electronics is reduced by the low-pass filtration. Moreover, the total duration of an integration cycle depends upon the magnitude of the two charging currents. Therefore, when one of the input currents becomes very small, the total time for obtaining the data points is very long. The long integration cycle, as well as the reduction in the threshold frequency, make it apparent that the known system of evaluation electronics is not particularly suitable for recognizing points of incidence of EMR in rapid motion.